Elevator cab doors typically move back and forth on door guide tracks mounted on the elevator cab in order to selectively open and close the entrance from the halls to the cab. When the cab arrives at a hall landing where one desires to enter or exit the cab, the cab doors will selectively link up with the hall doors, and will provide the opening and closing forces for the hall doors. The cab doors (and therefore the hall doors) are driven by a drive assembly which is mounted on the cab assembly.
Present conventional door drive assemblies include a reversible electric motor which is secured to the roof of the cab assembly. The drive motor includes a drive shaft which is selectively rotated in opposite directions, which drive shaft is connected to sets of articulated arms which are also connected to the doors. The articulated arms are connected at one end to a rotor which is driven by the drive motor; and are connected at the opposite end to the doors. In order to move the doors between their open and closed positions with minimal vibration and noise, the articulated arms are typically connected to the doors at a location which is close to the center of gravity of the doors. This tends to suppress rocking motion of the doors on their guide tracks, but it requires relatively excessively long arms. The use of long connecting arms between the drive motor and the doors creates large torque moments on the motor. The motor will thus tend to twist in one direction during opening operation of the doors, and twist in the opposite direction during closing operation of the doors. The torquing forces imposed on the door-actuating motor will thus be imparted to the cab structure. In order to counteract the aforesaid torquing forces, the cab must be structurally reinforced so that the cab will not ultimately be weakened by continual operation of the elevator. Therefore, elevator systems using the aforesaid door operators must employ a structurally reinforced cab that results in increased cab weight and complexity. It would be desirable to utilize an elevator cab door drive system which does not impose substantial torquing forces on the cab during operation of the doors.